Weapons-obsessed Damon Smith stuffed a rucksack with explosives and deadly ball-bearing shrapnel before leaving it on a Jubilee Line train last October.

The bag had been wired to a £2 alarm clock from Tesco, which was timed to go off minutes later.

If the device had exploded, it would have been just as passengers were being evacuated from the platform in North Greenwich.

Opening the case, Jonathan Rees QC, prosecuting, said: "The body of the device that contained the explosive substance also contained a quantity of ball bearings, which the prosecution allege had been included to act as shrapnel with a view to increasing the destructive effect of the device."

Smith, a computer forensics student at London Metropolitan University, admitted creating and planting the device, but said it was a "prank", the court heard.